1. Field of the Invention
The present invention relates to novel positive photoresist compositions based upon certain novolak polymers and containing diazoquinone-type photosensitizers. More specifically the invention relates to positive photoresist compositions which provide high resolution and high resistance to flow at elevated temperatures.
2. Description of Related Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components, such as in the fabrication of integrated circuits and printed wiring board circuitry. In these processes, a thin coating or film of a photoresist composition is generally first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or copper plates of printed wiring boards. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam, ion beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes.
After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate In some processes, it is desirable to bake the imaged resist coating before this developing step. This intermediate step is sometimes called post-exposure bake or PEB.
Photoresist compositions may be negative-working or positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to a developing solution. Thus, treatment of an exposed negative-working resist with a developer solution causes removal of the non-exposed areas of the resist coating and the creation of a negative image in the photoresist coating, and thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the resist composition exposed to the radiation become more soluble to the developer solution (e.g. the Wolff rearrangement reaction of the photoactive compound occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working resist with the developer solution causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
Positive-working photoresist compositions are currently favored over negative-working resists because the former generally have better resolution capabilities and pattern transfer characteristics.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases and the like. This etchant solution or plasma gases etch the portion of the substrate where the photoresist coating was removed during development. The areas of the substrate are protected where the photoresist coating still remains and, thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining resist layer after the development step and before the etching step to increase its adhesion to the underlying substrate and its resistance to etching solutions.
Certain uses of photoresists require that the photoresist formulations possess better lithographic properties for the fabrication of smaller microelectronic circuits. The lithographic properties which are critical to these uses include the following: (1) good resolution capabilities in both the micron and submicron ranges without incomplete development in the exposed areas (i.e. scumming); (2) higher thermal image deformation temperatures (e.g. above 120.degree. C.); (3) relatively fast photospeeds; (4) good adhesion to the substrate; (5) good developer dissolution rates; (6) good process latitute; (7) near to absolute vertical profiles (or good contrast) between exposed and unexposed photoresist areas after development; (8) good resistance to etching solutions and plasma etching techniques; (9) reduced tendency to form insoluble particulates and (10) mask linearity.
Generally, past efforts to improve one of these lithographic properties have resulted in significant decreases in one or more of the other lithographic properties of the photoresist. Accordingly, there is a need for improved photoresist formulations which possess all of these desired properties. The present invention is believed to be an answer to that need.
It is known to prepare positive photoresist compositions from novolak polymers and photosensitizers such as naphthoquinone diazide, such novolak polymers being formed by reacting a combination of m-cresol and p-cresol with formaldehyde in the presence of an acid catalyst. However such positive photoresist compositions do not demonstrate high resolution with high thermal resistance, due to the formation of isomer and oligomer mixtures as undesirable by-products. Reference is made to U.S. Pat. No. 4,529,682 (Hunt Chemical).
A large number of dimethyl phenol novolak compositions are known in the art. Reference is made to Japanese published patent applications J 60176034-A (JSR); J 60158440-A (Mitsubishi Chemical) and JP 62/89042-A2 (Mitsubishi Chemical). These references disclose the incorporation of p-cresol and/or m-cresol into their compositions as required ingredients. Such cresols result in the formation of by-products including isomers, such as p-cresol dimer, and oligomers, which reduce the resolution and thermal resistance of the compositions.